It has long been a desire of the cement concrete industry to place as much inexpensive pozzolan in a concrete mix as possible to minimize the amount of expensive Portland cement and not lose any of the attributes of the concrete such as setting time, initial strength, final hardness and durability. Specifically it is a goal to achieve concrete replacing as much Portland cement with supplementary cementitious materials such as fly ash slag or other materials as possible while minimizing or eliminating the use of Portland cement.
The goal is to make 100 slag grade or better concrete or 1157 performance cement using materials less costly than Portland cement. Regardless of the cost of pozzolan and Portland cement, over the years it has been found that pozzolan concrete has the best performance record as compared with Portland concrete, with the pozzolan concrete out-performing the Portland concrete with regard to sulfate attacks and alkaline silicon reactivity (ASR) attacks.
It will be appreciated that fly ash is part of the pozzolan family and that when one can replace Portland cement with as much as 20% Class F fly ash or 30% Class C fly ash, one obtains a very durable high strength concrete at considerably less than the price of Portland cement concrete
Specifically if it were possible to utilize granulated blast furnace slag, then it has been found that one can replace 50 to 80% of the Portland cement with this granulated blast furnace slag. However, granulated blast furnace slag is not always available or is too costly. If one has granulated blast furnace slag available the strength of the concrete approaches or equals ASTM C989 slag grade 100, whereas using untreated fly ash pozzolan has difficulty for qualifying for slag grade 80.
It will be appreciated that slag grade 100 refers to the fact that in 7 days one has achieved 75% strength for the Portland cement concrete, whereas in 28 days one achieves 95% strength.
It is therefore important to be able to process fly ash pozzolan to react on a level equal to that or better than that of granulated ground blast furnace slag.
In order to achieve 100 slag grade performance, or even 120 slag grade performance, the first requirement is to get the overall particle size distribution of the Class F fly ash down to under 45 microns, meaning that 98% or more of the Class F fly ash is under 45 microns in diameter. A better indicator is surface area. The surface area that is typical for fly ash pozzolan is around 0.695 m2/g. If it were possible to treat/mill pozzolan having a surface area of 0.695 m2/g to increase its surface area to 0.914 m2/g one could achieve slag grade 100 performance, or grade 120 slag pozzolan with around a 1.263 m2/g surface area or higher Given that various fly ashes have different surface areas one must move the surface area from its beginning surface area to increase it by a minimum of around 38% but preferably over 90% or higher from the parent sample of pozzolan including the additives.
How this is accomplished with the subject mill will be discussed hereinafter. As will be seen, the surface areas measured using a Beckman Coulter SA 3100 compressed helium/nitrogen Single Point BET analyzer show a significant increase when using the subject rotary mill. Moreover, when the pozzolan is treated in the subject rotary mill and when Limestone power station fly ash was tested using ASTM C 989 testing, the 0.693 m2/g Surface Area (SA) pozzolan yielded Slag Grade 80 performance; whereas 0.914 m2/g yielded Slag Grade 100 performance and the 1.263 m2/g yielded Slag Grade 120 performance when all were mixed with the same additive package as described hereinafter.
When one obtains ground fly ash at around 0.9 m2/g surface area (SA) one has components in the fly ash that are either reduced in particle size or the surface area is roughed up to increase its reactivity, thus allowing more reactive pozzolans to get into the chemical reaction of the concrete. Generally 80 micron or larger unground particles do not react unless they are ground down. It is generally accepted that the lower the pozzolans fineness (−45 micron) the more reactive the overall pozzolan will be while still meeting the ASTM C618 requirements.
Pozzolans come unground and consist generally of aspherical particles and spherical particles in the form of alumino ferro silicate glass beads. The treatment to be described both reduces the size of the non-spherical particles while at the same time roughing up the spherical particles. This allows for a much higher surface area with no reduction in the flow ability of the pozzolan and results in a concomitant rise in reactivity, going from a base of not meeting Slag Grade 80 performance to Grade 120 performance for a class F fly ash.
The performance used to measure class f fly ash is ASTM C618 which usually measures the pozzolanic activity index by testing a 20% pozzolan to cement mixture versus the slag requirement of a 50% pozzolan to cement mixture. Unless the pozzolan is treated using the subject process untreated fly ash pozzolan usually cannot pass Slag Grade 80 testing requirements.
It is thought that the smaller the particle size the higher the reactivity. Moreover, if fly ash can be ground down to under 45 microns one can air classify the material to select out the finer particles. In the air classification process one takes out the oversized particles, leaving the finer particles to provide the chemical reaction. However by having to remove the larger particles and dispose of them there is wastage, both in terms of material and energy expended. It would therefore be desirable to be able to grind down all of the Class F fly ash or pozzolan such that over 90% under is 25 microns, or if one could not grind down the fly ash sufficiently one would nonetheless like to be able to activate the surface of the fly ash to be able to make quality concrete.
While those in the cement/concrete industry have utilized large roll mills and ball mills for crushing pozzolans, these mills do not create activated particles both because merely crushing pozzolan does not polish or grind it and because the residence time in such mills is quite short. Typically in the crushing of pozzolans, the residence time is between 3 and 10 seconds because these types of mills are not grinding on the surfaces of the particles but rather splitting them apart by impact of the pozzolan particles on a rib or other projection where the particle is broken apart. Thus, the actual work that is being done on the surface of a particular particle is exceedingly short because those utilizing the large ball mills are simply trying to break apart the larger particles and not focusing on providing reactivity enhanced smaller particles having increased surface areas. Thus even if the larger ball mills could produce particles under 45 microns in diameter they nonetheless do not provide activated particles that the subject treatment to be described does. Moreover, chemical additives that are put in the process help activate the now receptive higher surface area pozzolan.
In short, all of the pozzolan milling to date has concentrated on fracturing the particles and not grinding them down or polishing them.